Drivers of Change
Greenland and Antarctica
Ice loss near the poles is one of the most critical changes pushing sea levels higher, a conclusion supported by data of increasing weight and accuracy. Greenland’s contribution to global sea-level rise is the largest, and increases every decade. Studies suggest that its melt grew from 0.09 millimeters per year between 1992 and 2001, expressed as the global sea-level rise equivalent, to 0.59 millimeters per year between 2002 and 2011 [Velicogna et al, 2014].
Measurements by the twin GRACE satellites (Gravity Recovery and Climate Experiment) show that most of the losses between 2003 and 2013 were coming from the southeast and northwest portions of the island, while the southwest is responsible for more than half of the acceleration of ice loss. The estimated total loss is in the range of more than 200 to more than 300 gigatons per year (1 gigaton is approximately 264 billion gallons of water. Melting 365 gigatons of ice would add 1 millimeter to global sea level; there are 25.4 millimeters in an inch). It is essential to understand, particularly on these short time scales, what part of the mass loss is due to changes in precipitation and surface melting and what part to changes in glacial discharge.
The measurements show that the pace of ice loss in Antarctica, while more moderate, remains sizable. Although East Antarctica has little mass loss, West Antarctica’s is significant. The Amundsen Sea region and the Antarctic Peninsula, both in West Antarctica, account for 64 percent of the total, some 180 gigatons per year between 2003 and 2013 (a loss offset by mass gains in East Antarctica, for a total loss for the continent of 67 gigatons per year [Velicogna et al, 2014]). And the Amundsen Sea area was the dominant contributor to the acceleration of ice loss, which increased about 11 gigatons each year.
Antarctica’s contribution to sea-level rise increased from 0.08 millimeters per year between 1992 and 2001 to 0.40 millimeters per year between 2002 and 2011 [Velicogna et al, 2014]. Together, Greenland and Antarctica contribute about one third of present-day sea level rise [Chen et al., 2013].
A 2012 study relying on altimetric, interferometric and gravimetric satellite data, as well as modeling [Shepherd et al., 2012], found that the Greenland ice sheet lost 142 gigatons per year between 1992 and 2011, though with an uncertainty of 49 gigatons per year. The same study saw 71 gigatons of ice loss in Antarctica, also with a large uncertainty factor. That adds up to a polar ice-sheet contribution of about 0.59 millimeters of sea-level rise per year for the study period.
And a recent reprocessing of GRACE data [Watkins et al., 2015] found 289 gigatons per year of ice-mass loss for Greenland between 2002 and 2014, and 141 gigatons for Antarctica.
Another study [Rignot et al., 2014] found a rapid rate of retreat for Amundsen Sea glaciers between 1992 and 2011, with their grounding lines, which separate ice on bedrock from floating ice, receding from 10 to 35 kilometers. These authors concluded that the ice retreats along regions of “retrograde bed elevation”—where the bedrock slopes downward, and farther away from the grounding line, in the inland direction. Ice-sheet numerical models find this configuration to be unstable.
Yet Antarctica illustrates the ability of broad-scale averaging to mask highly variable rates of change across regions. Some parts of the frozen continent, shielded by isolation and deep cold, are seemingly impervious to global warming—at least for the present. Queen Maud Land in East Antarctica even appears to be gaining ice mass—some 63 gigatons per year from 2003 to 2013 [Velicogna et al, 2014]. While not enough to overcome the continent’s net loss of ice, such gains do show that some regions can manage a shift toward higher ice mass, due to greater precipitation and fewer losses.
Glaciers and ice caps
Measurement of the Earth’s many glaciers and ice caps—smaller ice masses that are not a part of the Greenland or Antarctic ice sheets— show accelerated retreat. Together, they also account for about a third of the present sea level rise planet-wide (between 2.6 and 2.9 millimeters per year over at least the past 20 years). That places these smaller ice masses in the top three contributors to sea level rise, along with the warming of ocean water, which causes it to increase in volume, and the melting of the great ice sheets of Greenland and Antarctica. The largest losses are from Arctic Canada, Alaska and coastal Greenland, but with significant contributions from other regions, such as the Andes in southern Chile, Argentina or the Himalayas [Gardner et al., 2013].
Yet here, too, we encounter unexpected nuance as we move from bigger to smaller scales. While these smaller ice masses will remain among the dominant contributors to sea level rise through the next two centuries, their contribution likely drops to insignificance beyond the 200-year mark. Despite their outsized effects at present, glaciers and ice caps represent only about one percent of the Earth’s ice total (about 0.7 millimeters of equivalent sea level rise per year). So, once they are depleted, the great ice sheets become the overwhelmingly dominant contributors to ice loss.
Even if we confine our consideration to present-day losses, however, counter-intuitive findings emerge from broader statistical trends. The same research effort [Gardner et al., 2013] matched ground-based measurements of glacial retreat, and the corresponding contribution to sea level rise, with satellite measurements. While the researchers found that the individual glacial losses seen from the ground did indeed match those seen from orbit, they also revealed an unforeseen discrepancy. Estimates of total glacier-related ice loss, extrapolated from ground measurements, overstated those losses by a substantial margin.
The traditional method of measuring glacial retreat involves the yearly placement of marked poles in about 300 glaciers worldwide, tracking ice depletion over decades. Those measurements are used to extrapolate similar effects for the world’s entire complement of glaciers, some 160,000. For the period covered by the study—2003 to 2009, when both satellite and ground measurements were available for comparison—the ground-based method, known as the Randolph Glacier Inventory, yielded an estimated ice loss of 329 gigatons per year, plus or minus 121 [Gardner et al., 2013].
But the satellite observations offered a far different figure: 259 gigatons per year, plus or minus 28. That number was derived from data from two types of satellites, GRACE and ICESat.
Although the researchers did not fully investigate the reason for the discrepancy, they raised the possibility that the glaciers that were more easily accessible for ground measurements, at lower elevations, also happen to be melting the fastest—thus biasing extrapolations of total ice loss.
Another subtlety that must be accounted for involves feedback systems that, in some cases, amplify the warming and melting effects. The 2009 eruption of an Alaskan volcano, Mt. Redoubt, spread enough ash to darken area glaciers. Darker snow and ice absorbs more solar radiation, which accelerates melt. And indeed, the glacial ice-melt accelerated. [Arendt et al., 2013].
The warming of Earth is primarily due to accumulation of heat-trapping greenhouse gases, and more than 90 percent of this trapped heat is being absorbed by the oceans. Water volume rises with temperature because of thermal expansion—another major driver of sea level rise. The estimated rate of thermal expansion, or thermosteric sea level rise, from 1971 to 2010 is 0.4 to 0.8 millimeters per year; the estimate carries a confidence level of 90 to 100 percent [Rhein et al., 2013]. This corresponds to a warming rate of 0.015 degrees Celsius per decade in the upper 700 meters of the global ocean between 1971 and 2010. By comparison, an estimate using Argo floats found the thermosteric component of sea level rise above a depth of 2000 meters to be 0.5 millimeters per year, plus or minus 0.5 millimeters, between January 2005 and September 2010 [Leuliette and Willis, 2013]. Temperature measurements of the sea surface, taken by ships, satellites and drifting sensors, along with subsurface measurements and observations of global sea-level rise, lead researchers to conclude that this warming of the upper ocean over four decades is virtually certain. A contribution to sea level rise of about 0.1 millimeter per year by warming of ocean waters at depth, 700 meters to 2,000 meters, is considered likely, with about another 0.1 millimeter caused by warming deeper than 2,000 meters also considered likely [Rhein et al., 2013].
The expansion record is a short one; before 1971, ocean measurements were too few to allow meaningful estimates. Still, the record is robust enough to reveal that temperature-driven changes in seawater volume vary seasonally, as well as across decades. Combined with seasonal movement of rainfall, which brings shifts in water mass, such changes can cause sea levels in a given hemisphere to fluctuate by amounts approaching a centimeter [Chen et al., 2005].
The magnitude of expansion’s effect leads some researchers to urge adoption of sea level rise as the true “global warming number.” While we might instinctively look to globally averaged, surface air temperature as the benchmark of climate change, the sea level signal, these scientists argue, closely tracks the vast majority of the planet’s heat absorption.
Postglacial rebound, self-attraction and loading
The basins that hold Earth’s oceans do not remain static. Over long time scales they shift and deform in response to powerful forces. Because these, too, can alter sea levels, scientists must tease apart their effects from those of melting ice and thermal expansion.
One of the most important of these is post-glacial rebound, also known as glacial isostatic adjustment (GIA). At the close of the last ice age, some 10,000 years ago, the retreat of massive ice sheets from North America and the Eurasian continent lightened the load on the underlying mantle, deep below the Earth’s surface. The mantle is viscous, not solid, so it rebounds slowly, raising the rock layer above—the lithosphere—over thousands of years. This gradual lift, the recovery from the last ice age, continues today, altering the shape of ocean basins. And these alterations widen the basins, lowering apparent sea level by about 0.3 millimeters per year [Douglas and Peltier, 2002]. Researchers must factor this effect into their calculations when estimating the rate of sea level rise due to climate change.
Modern day melting also has a “rebound effect,” also called GIA.
A second effect, called ‘loading,’ is simple to understand: regardless of mantle viscosity, the lithosphere acts a bit like an elastic plate over large distances and short time scales, hence removing the weight of ice allows it to bounce back elastically. The same effect makes the seafloor go slightly up and down as tides move massive amounts of water from one location to another. While GIA acts over long time scales, ‘loading’ is a short-term effect [Tamisiea et al., 2010].
Another subtle effect can lower sea levels locally, even as the amount of water increases. The loss of ice mass means a loss of gravitational attraction; less surrounding water is pulled toward the ice, causing sea levels in the immediate vicinity to drop. The effect is counterintuitive: sea levels in the immediate neighborhood of melting Greenland do, in fact, go down. This effect is called “self attraction.”
Postglacial rebound and changes in loading are but two of the factors that can alter sea level by altering the elevation of the land. Excessive withdrawal of groundwater, for example, can cause the land surface to subside, raising coastal sea levels by comparison. Such interplay of effects might quickly lead to a measurement problem if scientists did not carefully calibrate their observations. They begin by drawing a sharp distinction between absolute and relative sea level—that is, sea level with respect to Earth’s center versus sea level with respect to the coast.
The Earth cycles water annually between land and ocean, in regional patterns of precipitation and evaporation, and this cycle also may be accelerating in response to climate change—with more variability, more flooding and more frequent drought.
Human manipulation of this water cycle, including increasingly high levels of groundwater withdrawal, contributes to sea level rise; the water withdrawn is ultimately reclaimed by the sea. And while the contribution is smaller than that of thermal expansion and melting ice, it is not insignificant.
Two recent attempts to isolate the sea level contribution of groundwater depletion [Konikow, 2011, Wada et al., 2012] yielded differing ranges, but by averaging the two, the most recent IPCC assessment estimated contributions to global mean sea level of 0.11 millimeters per year from 1901 to 1990, 0.12 from 1971 to 2010, and 0.38 from 1993 to 2010. This compares to the assessment’s overall sea-level rise estimate of 1.7 millimeters per year from 1901 to 2010, and 3.2 from 1993 to 2010 [Church and White, 2011].
Sharpening the accuracy of global sea-level rise estimates will require a clearer picture of other land-based influences on water storage and transfer. These include human engineering, such as the construction of reservoirs for water storage or dams to hold water back, which can tip the balance toward water storage on land (though the effects of groundwater depletion now surpass those of reservoir storage, keeping that balance tipped toward the ocean). The potential consequences of groundwater pumping that can raise sea levels by bringing land elevation down, the draining of wetlands and the conversion of habitat types also remain difficult to assess, but hold some of the keys to a more precise characterization of land-based contributions to sea level rise.
As with so many other attempts to capture sea level data, firmly establishing input from the land will depend upon satellite measurements, including gravitational monitoring by the twin GRACE satellites.